1. Technical Field
The present invention relates to electrophotographic image forming apparatuses that perform either copier, printer or facsimile machine functions, or to multifunctional apparatuses that perform all of these functions, and to transfer units that are employed for these image forming apparatuses.
2. Related Art
In an electrophotographic image forming apparatus such as a copier or a printer, for example, a developing device (a developing unit) employs a developer to form an electrostatic latent image on a photosensitive drum, and then, the toner image produced on the photosensitive drum is transferred and fixed to a recording medium, conveyed by a sheet feeding unit, to obtain a printed image. Some image forming apparatuses of this type include a belt conveying device, such as an endless intermediate transfer belt, a photosensitive belt or a sheet conveying belt. More specifically, intermediate transfer belts are provided for certain color image forming apparatuses, of a tandem type, that include image forming units for the individual colors yellow, magenta, cyan and black.
Generally, for a belt drive unit for which, to support an endless belt, there are provided a predetermined number of rollers, certain of which are employed to drive the endless belt, the endless belt, while being driven, is shifted width wise (perpendicular to the direction in which the belt is being driven), i.e., a so-called belt walk phenomenon (belt skewing) occurs. This belt walk phenomenon causes the positions of individual color images to be shifted when the tandem color image forming apparatus transfers and superimposes these images on the intermediate transfer belt. That is, belt walk causes misregistration and the uneven application of image colors. Therefore, in order to produce a high quality image, belt walk must be prevented.